1. Field of the Invention
The present invention relates to a fixing device for fixing a toner image onto a recording medium such as a recording sheet, and an image forming apparatus including the fixing device.
2. Description of the Related Art
An image forming apparatus for forming images on a recording sheet by an electrophotographic method, as a copier, a fax machine, and a printer, charges a photoreceptor with a charging device, and then forms an electrostatic latent image on the surface of the photoreceptor by optical writing according to an original image or image information. This electrostatic latent image is turned into a toner image that is a visible image, by using toner of a developing device. This toner image is fixed onto a recording sheet as a fixing device applies heat and pressure onto the recording sheet, thereby obtaining a copy or a printout.
Presently, there are two methods used as a fixing device used in image forming apparatuses. One method is a roller method including a heating roller with a heat generating unit provided inside and a pressurizing roller that rotates by contacting the heating roller. A recording sheet is conveyed between the heating roller and the pressurizing roller, and heat and pressure is applied to the recording sheet to fix a toner image on the recording sheet. The other method is a belt method in which an endless fixing belt is wound around heating rollers provided with a heat generating unit. A recording sheet is conveyed between the fixing belt and a pressurizing roller, and heat and pressure is applied to the recording sheet to fix a toner image on the recording sheet. Furthermore, there are cases where the heat source is placed directly on the inside of the fixing belt.
Image forming apparatuses are desired to be highly productive and power-saving. Furthermore, image forming apparatuses are desired to have the capability of handling recording sheets of various thicknesses and sizes. Particularly, in a case of continuously passing through recording sheets made of cardboard while maintaining high productivity, the heat energy stored in the fixing device is consecutively taken away. Therefore, for the purpose of maintaining fixing properties, the heat generating source needs to be energized to apply heat so that the lost heat is replenished.
A fixing device using an infrared heater as the heat generating source is desired to quickly heat the heating roller by increasing the lighting rate of the infrared heater. Furthermore, in the case of a sheathed heater, the energizing rate needs to be increased. Here, the lighting rate and the energizing rate refer to the ratio of the power supply amount applying the actual heat generation amount, with respect to the power supply amount corresponding to the maximum heat generation capability.
In order to reduce the power consumption of a fixing device, unnecessary energization needs to be eliminated. In a case where a recording sheet having a narrow width is passed through the fixing device, for the purpose of eliminating any wasteful consumption of heat energy, the heat generating unit is divided in the sheet width direction, and the heating time of the heating roller is controlled in accordance with the width of the recording sheet (Patent Document 1).
Recently, there is used a fixing device in which the heating roller is made thin and the heat capacity of the heating roller is decreased, for the purpose of increasing the temperature rising rate. In recent years, there has been demand for high-speed printing operations, and therefore a large amount of heat of the heating roller is taken away by the sheets, which tends to cause an even larger temperature difference between the sheet passing part (part where the sheet passes) and the sheet non-passing part (part where the sheet does not pass). This is particularly significant in a fixing device in which the heating roller is made thin and the heat capacity of the heating roller is decreased for the purpose of increasing the temperature rising rate.
In order to compensate for the heat taken away by the sheets, the heater needs to be lit at a higher lighting rate. Accordingly, at the sheet non-passing part, the heat energy becomes excessively high and the temperature rises. In the worst case, the temperature exceeds the heatproof temperature of the heating roller (belt). Consequently, serious failures may occur, such as the deformation of a member.
In order to control the temperature rise due to heat, detailed temperature control operations are needed for each heat source, and therefore the surface temperature of the fixing device needs to be known in detail. Thus, when there are a plurality of heat sources, the number of temperature sensors increase according to the number of heat sources. However, an increase in the number of temperature sensors leads to an increase in cost. Thus, there is disclosed an example in which the temperature sensors are positioned such that the number of temperature sensors can be reduced (Patent Document 2).
Sheets are not always passed through the fixing device. Therefore, the contents of control operations of the heat sources differ between the time of warning up, i.e., when the power is first turned on and the temperature of the fixing device is raised, and the time of standby while waiting for the printing to start. There is disclosed an example of controlling the heat generation amount according to the state of the fixing device, because in the standby state while waiting for the printing to start, the necessary amount of heat generation decreases (Patent Document 3).
However, by the conventional method, power saving is not sufficiently implemented in the fixing device.
Patent Document 1: Japanese Examined Patent Application Publication No. S63-44223
Patent Document 2: Japanese Laid-Open Patent Publication No. 2003-272802
Patent Document 3: Japanese Patent No. 4687043